Merging two dataframes only at specific times

I have two excel files that I'm trying to merge into one using

pandas

. The first file is a list of times and dates with a subscriber count for that given time and day. The second file has weather information on an hourly basis. I import both files and the data resembles:

I've managed to test the code that I've written for a one month period and the problem that I'm encountering is that it is taking a great deal of time to complete the task. I wanted to know if there was a faster way of going about this